Gold(I) catalysts with bifunctional P, N ligands

Article abstract of DOI:10.1021/ic2011259

A series of phosphanes with imidazolyl substituents were prepared as hemilabile PN ligands. The corresponding gold(I) complexes were tested as bifunctional catalysts in the Markovnikov hydration of 1-octyne, as well as in the synthesis of propargylamines by the three component coupling reaction of piperidine, benzaldehyde, and phenylacetylene. While the activity in the hydration of 1-octyne was low, the complexes are potent catalysts for the three component coupling reaction. In homogeneous solution the conversions to the respective propargylamine were considerably higher than under aqueous biphasic conditions. The connectivity of the imidazolyl substituents to the phosphorus atom, their substitution pattern, as well as the number of heteroaromatic substituents have pronounced effects on the catalytic activity of the corresponding gold(I) complexes. Furthermore, formation of polymetallic species with Au₂, Au₃, and Au₄ units has been observed and the solid-state structures of the compounds [(5)₂Au ₃Cl₂]Cl and [(3c)₂Au₄Cl ₂]Cl₂ (3c = tris(2-isopropylimidazol-4(5)-yl phosphane, 5 = 2-tert-butylimidazol-4(5)-yldiphenyl phosphane) were determined. The gold(I) complexes of imidazol-2-yl phosphane ligands proved to be a novel source for bis(NHC)gold(I) complexes (NHC = N-heterocyclic carbene).

Full text of DOI:10.1021/ic2011259

ARTICLE
pubs.acs.org/IC
Gold(I) Catalysts with Bifunctional P, N Ligands
Corinna Wetzel,† Peter C. Kunz,*,† Indre Thiel,† and Bernhard Spingler‡
†Institut f€ur Anorganische Chemie und Strukturchemie I, Heinrich-Heine Universit€at D€usseldorf, Universit€atsstrasse 1,
D-40225 D€usseldorf, Germany
‡Institute of Inorganic Chemistry, University Zurich-Irchel, Winterthurerstrasse 190, CH-8057 Z€urich, Switzerland
S Supporting Information
b
ABSTRACT: A series of phosphanes with imidazolyl substit-
uents were prepared as hemilabile PN ligands. The correspond-
ing gold(I) complexes were tested as bifunctional catalysts
in the Markovnikov hydration of 1-octyne, as well as in the
synthesis of propargylamines by the three component cou-
pling reaction of piperidine, benzaldehyde, and phenylacety-
lene. While the activity in the hydration of 1-octyne was low, the
complexes are potent catalysts for the three component cou-
pling reaction. In homogeneous solution the conversions to the
respective propargylamine were considerably higher than under
aqueous biphasic conditions. The connectivity of the imidazolyl
substituents to the phosphorus atom, their substitution pattern,
as well as the number of heteroaromatic substituents have
pronounced effects on the catalytic activity of the corresponding gold(I) complexes. Furthermore, formation of polymetallic
species with Au2, Au3, and Au4 units has been observed and the solid-state structures of the compounds [(5)2Au3Cl2]Cl and
[(3c)2Au4Cl2]Cl2 (3c = tris(2-isopropylimidazol-4(5)-yl phosphane, 5 = 2-tert-butylimidazol-4(5)-yldiphenyl phosphane) were
determined. The gold(I) complexes of imidazol-2-yl phosphane ligands proved to be a novel source for bis(NHC)gold(I)
complexes (NHC = N-heterocyclic carbene).
’ INTRODUCTION
relevance and their importance as building blocks in the pre-
paration of nitrogen-containing molecules and as key intermedi-
ates for natural product synthesis.11 For these syntheses, in
addition to gold(I) and gold(III) compounds,12 iron13 and
indium14 salts have been employed as catalysts.12 The functio-
nalization of hydrocarbons is another important area for catalytic
transformations. Here the hydration of terminal alkynes to the
respective ketones (Markovnikov product) or aldehydes (anti-
Markovnikov product) is of particular interest. Gold(I) species
have started to replace gold(III) complexes,15 since Teles et al.
showed that complexes of the general composition [(L)Au]+,
with L being a phosphane, arsane, or phosphite ligand, are very
good catalysts for the addition of alcohols to alkynes, if an
acid cocatalyst is present.16 In the past few years gold(I)ÀNHC
(N-heterocyclic carbene) complexes17 havestartedtogainimmense
interest and many catalytic active gold(I) species concerning the
hydration of alkynes have been described.18 Nonetheless there
are several recent examples of gold(I)Àphosphane complexes19
including gold(I) complexes with bifunctional pyridylphosphane
ligands, which readily convert 1-pentyne to 2-pentanone.20
For a long time, gold has been regarded as inactive as a
catalytic metal because of its “chemical inertness”. However,
recent work on the high catalytic activity of gold compounds in
heterogeneous and homogeneous catalysis proved the opposite.1
Gold-catalyzed CÀC coupling reactions provide excellent meth-
ods for the construction of complex molecules under mild
conditions.2 First, saltlike gold compounds, such as AuCl3 or
HAuCl4, were used as catalysts followed by gold complexes
bearing phosphane ligands.3
Pyridylphosphanes are well-established PN ligands in transi-
tion metal chemistry.4 Imidazole-based phosphane ligands
are, however, less studied. The soft phosphorus and harder nitro-
gen atoms of those ligands make them potentially hemilabile5
and bifunctional.6 A variety of reactions including CÀC bond
formation,7 carbonylation of amines,8 asymmetric aldol reactions,9
and hydration of terminal alkynes10 have been reported using
gold(I) catalysts with bifunctional PN ligands. An interesting
reaction is the CÀC bond formation in the multicomponent
coupling reaction of an aldehyde, amine, and alkyne to the
respective propargylamine. One-pot multicomponent coupling
reactions are efficient methods for the preparation of complex
molecules starting from readily available materials.
In this work, we describe the synthesis and structural pro-
perties of novel gold(I) complexes, containing bifunctional
The synthesis of propargylamines has attracted considerable
attention over the past few years because of their pharmaceutical
Received: May 26, 2011
Published: July 15, 2011
r
2011 American Chemical Society
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|

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ARTICLE
Scheme 2. Reaction Scheme for the Synthesis of Complexes
of the Type (L)AuCl
Scheme 3. Three-Component Coupling of Benzaldehyde,
Piperidine, and Phenylacetylene Catalyzed by Gold(I)
Complexes
Figure 1. PN ligands used (a) imidazol-2-yl phosphanes R = H (1), Me
(2), (b) imidazol-4(5)-yl phosphanes R = iPr (3), Ph (4), tBu (5) {n =
2À0 (aÀc)}, and (c) 1-methylimidazol-4-yldiphenylphosphane (6) and
1-methylimidazol-5-yldiphenylphosphane (7).
Scheme 1. Reaction Schemes for the Syntheses of Imidazol-
4(5)-yl phosphanes 4-MIPR (R = iPr, Ph, tBu)
In contrast to the well-elaborated procedures for the pre-
paration of imidazol-2-ylphosphanes, syntheses of the isomeric
imidazol-4-yl- and imidazol-5-ylphosphanes are only scarcely
described.21 The imidazol-2-ylphosphanes 1a,c, 2a,c, 3aÀc, 6,
and 7 were prepared according to literature procedures.22 For
the imidazol-4(5)-ylphosphane ligands we adopted a protocol
previously described by us starting from 2-organylimidazoles
(Scheme 1).23 The 31P{1H} NMR spectroscopic data of synthe-
sized ligands is summarized in Table 1 and the 31P{1H} NMR
chemical shifts are typical for mono-, bis-, and tris(imidazolyl)
phosphanes.
Table 1. Spectroscopic Data of the Ligands and Gold(I)
Complexes (Δδ = δC À δL)
The gold(I) phosphane complexes of the type [(L)AuCl]
(L = imidazolyl phosphane ligand) are obtained as white solids in
good yields by reaction of the corresponding ligand and [(tht)-
AuCl] in dichloromethane or methanol at room temperature
(Scheme 2). The complex [(6)AuCl] was prepared in acetone as
both the tris(imidazol-2-yl)phosphane ligands and the corre-
sponding gold(I) complexes tend to decompose in protic
solvents. The decomposition of all gold(I) complexes bearing
imidazol-2-yl phosphane ligands was monitored by time depen-
ligand
δL(31P)
δC(31P)
Δδ(31P)
MIPR
1a
2a
3a
10
11
12
4
À22b
À28b
À31b
À25b
À34a
À29a
À34b
À31a
À46b
À45b
À58b
À73c
À59c
À80b
16b
13a
14b
11a
11a
11a
11c
12a
À4b
À4a
À5b
À20c
À19c
À23b
38
41
45
36
45
40
45
43
42
41
53
53
40
57
1
dent H NMR spectroscopy in methanol-d4 and D2O (see
Catalyst Deactivation Pathways and Supporting Information).
1
All complexes were characterized by H and 31P{1H} NMR
5
BIPR
TIPR
1b
2b
3b
1c
2c
3c
spectroscopy, as well as MALDI and ESI mass spectrometry and
elemental analysis (see Supporting Information). The 31P{1H}
NMR chemical shifts of the ligands and the resulting complexes
(L)AuCl are summarized in Table 1. The 31P{1H} NMR
spectra of the complexes show a substantial coordination shift
of 35 to 57 ppm to lower field compared to the free ligands.
Because of the ambidentate nature of the PN ligands, next to
the usual linear (kP) coordination pattern of gold, dinuclear
(k2P,N) structures can result. Previously, solid state structures of
the mononuclear complex [(3a)AuCl] and the dimeric structure
[{(3c)Au}2]Cl2 have been reported by us.24 A coordination shift
of Δδ(31P) > 50 ppm is usually found for a bridging coordination
mode of the PN ligand, whereas a shift of ∼40 ppm is found in
the mononuclear complexes. The MALDI MS spectra of all
complexes (L)AuCl show the signal for the ion [(L)AuCl]+ as
the basic peak and a further signal for the corresponding dimer
[(L)2Au2Cl]+.
a CDCl3. b MeOD-d4. c DMSO-d6.
imidazolylphosphanes. The new compounds were tested for
their application as catalysts in the synthesis of propargylamines
and the hydration of terminal alkynes. On the basis of the results
in the catalytic trials, we report different pathways for potential
deactivation of the (L)AuCl catalysts.
’ RESULTS AND DISCUSSION
In this work we used different homologous series of poly-
dentate imidazolylphosphanes (Figure 1) and investigated their
application as ligands in gold(I) catalyzed reactions.
Three-Component Coupling Reaction. Selected gold(I)
complexes with PN ligands have been investigated as potential
bifunctional catalysts in the synthesis of propargylamines by
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Table 2. Catalytic Trials of Complexes (L)AuCl in the Three-
Component Coupling of Phenylacetylene, Benzaldehyde, and
Piperidine under Biphasic Conditions (Conversion Deter-
mined by 1H NMR Spectroscopy, Given in Mean ( Error)
Scheme 4. Markovnikov Hydration of Alkynes
entry
ligand
catalyst loading [mol %]
conversion [%]
1
2
3
4
5
6
7
2-MIPNMe (2a)
2-BIPNMe (2b)
2-TIPNMe (2c)
2-TIPNMe (2c)
4-MIPiPr (3a)
4-BIPiPr (3b)
4-TIPiPr (3c)
0.5
0.5
0.5
5
63 ( 1
75 ( 5
65 ( 3
97 ( 1
16 ( 1
40 ( 2
37 ( 3
Table 4. Preformed Gold Complexes and Their Catalytic
Activity after Halide Abstraction in Situ Using AgOTf in
Acetone
entry
catalyst
conversiona [% (h)]
0.5
0.5
0.5
1
(1a)AuCl
6 (22)
30 (22)
2
(2a)AuCl
3
(2b)AuCl
5 (22)
4
(2c)AuCl
0 (22)
Table 3. Screening of Catalysts (L)AuCl in the Three-Com-
ponents Coupling of Phenyl Acetylene, Benzaldehyde, and
Piperidine under Homogenous Conditionsa
5
(6)AuCl
17 (22)
6
(3a)AuCl
2 (22)
7
(3b)AuCl
0 (22)
entry
ligand
catalyst loading [mol %]
conversion [%]
8
[{(3c)Au}2]Cl2
[{(3c)Au}2{AuCl}2]Cl2
[{(5)Au}2AuCl2]Cl
(4)AuCl
0 (22)
1
2-MIPNMe (2a)
4-MIPNMe (6)
5-MIPNMe (7)
2-BIPNMe (2b)
2-TIPNMe (2c)
2-TIPNMe (2c)
2-MIPH (1a)
2-BIPH (1b)
2-TIPH (1c)
4-MIPiPr (3a)
4-BIPiPr (3b)
4-TIPiPr (3c)
4-MIPtBu (5)
TPPMS
0.5
0.5
0.5
0.5
0.5
5
70 ( 2
61 ( 3
92 ( 1
82 ( 2
95 ( 2
98 ( 1
46 ( 1
74 ( 2
87 ( 1
26 ( 2
35 ( 4
74 ( 1
67 ( 2
25 ( 1
32 ( 5
39 ( 4
26 ( 3
9
1 (22)
2
10
11
12
60 (22)
3
61 (22), 93 (48)
95 (22)
4
[(PPh3)AuCl]
a Conversion to 2-octanone.
5
6
7
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
the so modified protocol are summarized in Table 3. The con-
versions are considerably higher compared to the results ob-
tained under aqueous biphasic conditions.
8
9
10
11
12
13
14
15
16
17
It is obvious that the connectivity (2-, 4-, or 5-yl), as well as the
substitution pattern of the imidazolyl substituents in the PN
ligands show an effect on the catalytic activity of their gold(I)
complexes. The catalytic activity of the complexes increases
within every series in the order MIP < BIP < TIP. Within the
series, the complexes with the N-methylated ligands 2a-c show
the highest activity. Complex (2c)AuCl shows the highest
conversion of 95% at a catalyst loading of 0.5 mol %. Here, the
conversion is as high as under biphasic conditions using the 10-
fold higher catalyst loading.
Within the series of the complexes (L)AuCl with the isomeric
ligands 2-MIPNMe (2a), 4-MIPNMe (6), 5-MIPNMe (7) (entries
1À3), (2a)AuCl, and (6)AuCl exhibit about the same conver-
sion (70 and 61%, respectively), whereas (7)AuCl shows a con-
version of 92%. All complexes, with exception of (3a)AuCl
(entry 10), show higher catalytic activity then the reference
compounds (entries 14À17).
TPPDS
TPPTS
Ph3P
a Na[(TPPMS)AuCl], Na2[(TPPDS)AuCl], Na3[(TPPTS)AuCl], and
[(Ph3P)AuCl] as reference compound (conversion determined by H
NMR spectroscopy, given in mean ( error).
1
three component coupling of piperidine, benzaldehyde and
phenyl acetylene (Scheme 3).
First we investigated the catalytic activity of the selected
gold(I) complexes under biphasic conditions, as especially the
tris(imidazolyl)phosphane ligands are water-soluble. The cat-
alytic trials were carried out in aqueous reaction mixtures under
an atmosphere of N2 at 40 °C for 42 h using the procedure
published by Elie et al.25 The results of the trials under biphasic
conditions are summarized in Table 2. Complex (2b)AuCl
shows the highest activity with a conversion of 75% and a
catalyst loading of only 0.5 mol % but when the catalyst loading
was increased to 5 mol % even conversion by complex
(2c)AuCl raised from 65 to 97%. It has to be mentioned that
the complexes (L)AuCl of ligands 2a, 2b, 3a, and 3b did not
dissolve completely under the conditions used for the biphasic
catalysis (at 0.5 mol %).
Markovnikov Hydration of Terminal Alkynes. Since gold(I)
complexes with pyridylphosphane ligands have been shown to
convert 1-pentyne to 2-pentanone in very high yields,20 the
catalytic activity of our gold(I) imidazolylphosphane complexes
in the hydration of terminal alkynes was investigated as well
(Scheme 4). The respective gold(I) complex (5 mol % in regard
to the alkyne) was dissolved in degassed acetone, the chloride
abstracted in situ by addition of 1 equiv AgOTf. 1-Ocytne and
water (10 equiv in regard to the alkyne) were added to the
solution, which was then stirred at 60 °C. The results of the
catalytic trials are summarized in Table 4.
When the gold(I) complexes were used in the neat mixture of
the organic reactants the catalytic trials could be carried out under
homogeneous conditions (40 °C, 42 h). The results obtained by
While gold complexes with bis(imidazolyl)phosphanes, as
well as tris(imidazolyl)phosphanes of any kind, proved inactive,
complexes with mono(imidazolyl)phosphanes show catalytic
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Figure 2. Molecular structure of [(5)2Au3Cl2]Cl, ellipsoids at 30%
probability. Hydrogen atoms, solvent molecules, and noncoordinated
chloride anions were omitted for clarity. Selected bond lengths [Å] and
angles [deg]: Au1ÀAu2, 3.1863(4); Au1ÀAu3, 3.3075(4); Au2ÀAu3,
3.1043(4); Au2ÀN2, 2.013(5); Au2ÀN3, 2 2.017(6); Au1ÀP1,
2.236(2); Au1ÀCl1, 2.290(2); Au3ÀP31, 2.232(2); Au3ÀCl31,
2.294(2); Au1ÀAu2ÀAu3, 63.427(9); Au1ÀAu3ÀAu2, 59.495(9);
Au2ÀAu1ÀAu3, 57.078(9); N2ÀAu2ÀN32, 176.0(2); P1ÀAu1ÀCl1,
175.24(7); P31ÀAu3ÀCl31, 173.55(8).
Figure 3. Molecular structure of [(3c)2Au4Cl2]Cl2, ellipsoids at 50%
probability. Hydrogen atoms, disordered methyl groups, and noncoor-
dinated chloride anions were omitted for clarity. Selected bond lengths
[Å] and angles [deg]: Au1ÀAu2, 3.0695(4); Au1ÀAu2, 3.1102(4);
Au1ÀP1, 2.2258(19); Au1ÀCl1, 2.2848(19); Au2ÀN18, 2.019(6);
Au2ÀN2, 2.023(6); P1ÀAu1ÀCl1, 176.73(7); N2ÀAu2ÀN18,
174.1(2); Au1ÀAu2ÀAu1, 94.246(10); Au2ÀAu1ÀAu2, 85.754(10).
Scheme 6. Formation of Gold(I) Bis(NHC) Complexes
Scheme 5. Synthesis of Polynuclear Complexes Aun Stabi-
lized by PN Ligands
been mentioned as catalyst for the hydration of terminal alkynes
before.
Catalyst Deactivation Pathways. A major problem of multi-
functional ligands in catalytic applications is the elucidation of
the structure of the actually active species. In the case of gold(I)
complexes of PN ligands, for example, imidazolyl phosphanes, it
is hard to define which species is the catalytic active species and
which species are present in solution. Different coordination
modes as kP- or kN-monodentate, k2PN-chelating, μ-PN brid-
ging, as well as polynuclear species with combination of these
binding modes can be formed. Complexes bearing Au2, Au3, and
Au4 units have been observed with different imidazolyl phos-
phane ligands before.27 From the reaction mixture of trial 12,
Table 3, we obtained a crystal of [(5)2Au3Cl2]Cl whose structure
could be determined by X-ray diffraction (Figure 2). Such
polynuclear complexes can be prepared directed by reaction of
the PN ligands with [(tht)AuCl] in the corresponding stoichi-
ometry (Scheme 5). This has been demonstrated by the forma-
tion of complexes [(L)2Au3Cl2]Cl (L = 3c, 5) and complex
[(3c)2Au4Cl2]Cl2 (Figure 3). The Aun complexes (n = 2À4) of
the PN ligands show coordination shifts of Δδ(31P) > 50
ppm which is typical for a bridging coordination mode of the
PN ligand, as stated before. The polynuclear structure of these
complexes is seen by the characteristic signals of the molecular
ions in the MALDI-TOF spectra.
activity toward the hydration of 1-octyne. The most active
gold(I) imidazolyl complexes are [{(5)Au}2AuCl2]Cl and
[(4)AuCl] with 60% and 61% conversion of 1-octyne to
2-octanone within 22 h, respectively. N-Methylated imidazol-2-
yl ligands show a higher conversion rate than ligands containing a
NH-function and ligands possessing large substituents in the
position adjacent to the nitrogen show the highest conversion
rates (entries 1À4).
Interestingly, the triphenylphosphane gold(I) complex
[(PPh3)AuCl] shows the highest activity in the conversion of
1-octyne to 2-octanone (entry 12). Complexes of the type
[(PPh3)Au(CH3)] have already been shown to be good catalysts
in the hydration of alkynes in the presence of acid cocatalysts.
The thus generated [(PPh3)Au]+ is active in many reactions
involving triple bonds,26 one of them being the addition of
methanol across a triple bond. However, [(PPh3)AuCl] has not
On the basis of the results of the catalytic trials under biphasic
conditions and further stability studies, we observed that gold(I)
complexes of imidazol-2-ylphosphane ligands are unstable in
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Scheme 7. Protonation of Imidazol-2-phosphanes Give
Imidazolylphosphanes
imidazolyl substituents to the phosphorus atom, their substitu-
tion pattern, as well as the number of heteroaromatic substituents
have important effects on the catalytic activity of these gold(I)
complexes.
In the Markovnikov hydration of 1-octyne the gold(I) com-
plexes showed activities below or in the magnitude of [(Ph3P)-
AuCl]. Here the catalytic activity of complexes within the
imidazol-4-yl phosphane series increases with increase in steric
bulk of the substituent (t-Bu > Ph > i-Pr) at the C2-position,
which has been reported for pyridyl-based PN ligands.
The imidazole-based PN ligands can display various coordina-
tion modes in their gold(I) complexes. Depending on the ligand-
to-metal ratio polymetallic complexes with Au2, Au3, and Au4
units can be formed. A unique feature of imidazol-2-yl phos-
phanes within the isomeric imidazol-2-yl, 4-yl-, and -5yl phos-
phanes is their sensitivity toward PÀC bond cleavage in protic
solvents. This reactivity resembles the NHC-phosphenium ad-
duct nature of imidazol-2-yl phosphanes after protonation. In the
corresponding gold(I) complexes this reaction results in the
formation of catalytically inactive bis(NHC)Àgold(I) complexes
with imidazol-2-yl phosphane ligands as well as the formation of
polynuclear species with imidazol-4(5)-yl phosphane ligands.
protic solvents, especially complexes (1c)AuCl and (2c)AuCl. In
water and methanol solvolysis of the PÀC bond and therefore
decomposition of the gold complexes yields H3PO3 and P-
(OCH3)3, respectively, imidazole and the corresponding bis-
(NHC) gold(I) complexes [(C3H4N2)2Au]+ (C6) and
[(C4H5N2)2Au]+ (C3), respectively (Scheme 6).
The formation of complex C6 is completed within one hour.
Complex (2c)AuCl is more stable and the corresponding bis-
(NHC) complex C3 is formed within 20 h. The bis(NHC) gold(I)
complexes were identified by MALDI TOF MS, 1H and 13C{1H}
NMR spectroscopy. The NMR signals were identified unam-
biguously by adding samples of the independently prepared
bis(NHC)s gold(I) complexes [(C3H4N2)2Au]Cl28 (C6) and
[(C4H6N2)2Au]Cl29 (C3) to the corresponding reaction mix-
tures. Both bis(NHC) complexes C3 and C6 were tested for
their catalytic activity and show no activity in the three compo-
nent coupling reaction nor in the hydration of terminal alkynes.
The stability of imidazol-2-yl phosphane gold(I) complexes
toward solvolysis depends on the number of heteroaryl substit-
uents on the phosphorus atom. While the tris(imidazol-2-yl)pho-
sphane complexes (1c)AuCl and (2c)AuCl decompose within
minutes to the respective bis(NHC) gold(I) complexes, the
corresponding BIP and MIP complexes are much more stable in
protic solvents. This is in accord with the observed activity of the
corresponding complexes in the three component coupling
reaction under biphasic and homogeneous conditions (Table 2
and 3). In contrast to the gold(I) complexes of imidazol-2-
ylphosphane ligands, the isomeric compounds containing the
PÀC bond in C4- or C5-position of the imidazolyl substituent
are stable in protic solvents and do not decompose even after
prolonged time.
’ EXPERIMENTAL SECTION
The procedures for the preparation of ligands 4-MIPPh (4) and
4-MIPtBu (5) and complexes (L)AuCl, as well as the corresponding
NMR and MS data and elemental analyses can be found in the
Supporting Information. The compounds [(tht)AuCl], 1-methyl-2-
trimethylsilylimidazole, 2-MIPH (1a), 2-TIPH (1c),), 4-BIPiPr (3b),
4-MIPNMe (4), 5-MIPNMe (5), as well as the gold(I) complexes
(2a)AuCl, (2b)AuCl, (2c)AuCl, [(3a)AuCl], [(3c)2Au2]Cl2, Na[(tppms)-
AuCl], Na2[(tppds)AuCl], and Na3[(tppts)AuCl] were prepared ac-
cording literature procedures. The preparations were carried out in
Schlenk tubes under an atmosphere of dry nitrogen using anhydrous
solvents purified according to standard procedures. The metal com-
plexes were prepared using wet solvents. All chemicals were used as
1
purchased. H and 31P{1H} NMR spectra were recorded on a Bruker
DRX 200 and 13C{1H} NMR spectra on a Bruker DRX 500 spectro-
meter. The 1H and 13C{1H} NMR spectra were calibrated against the
residual proton signals and the carbon signals of the solvents as internal
references (chloroform-d, δH = 7.30 ppm and δC = 77.0 ppm; methanol-
d4, δH = 3.31 ppm and δC = 49.1 ppm; dmso-d6, δH = 2.50 ppm and
δC = 39.5 ppm; D2O, δH = 4.79), while the 31P{1H} NMR spectra were
referenced to external 85% H3PO4. The MALDI mass spectra were
recorded on a Bruker Ultraflex MALDI-TOF mass spectrometer and ESI
mass spectra with a Finnigan LCQ Deca Ion-Trap-API mass spectro-
meter. The elemental composition of the compounds was determined
with a Perkin-Elmer Analysator 2400 at the Institut f€ur Pharmazeutische
and Medizinische Chemie, Heinrich-Heine-Universit€at D€usseldorf.
Crystallographic data were collected at 183(2) K on an Oxford
Diffraction Xcalibur system with a Ruby detector using Mo KR radiation
(λ = 0.7107 Å) that was graphite-monochromated. Suitable crystals were
covered with oil (Infineum V8512, formerly known as Paratone N),
mounted on top of a glass fiber and immediately transferred to the
diffractometer. The program suite CrysAlisPro was used for data collec-
tion, multiscan absorption correction and data reduction.31 Structures
were solved with direct methods using SIR9732 and were refined by full-
matrix least-squares methods on F2 with SHELXL-97.33 The structures
were checked for higher symmetry with help of the program Platon.34
The structure of [(5)2Au3Cl2]Cl contained five chloroform molecules
out of which three are half occupied. Two of these three chloroform
molecules are sitting on special positions. Remaining residual electronic
Very recently, Chauvin et al. reported on the formation
of NHC complexes from imidazolylphosphane complexes by
PÀC bond cleavage.30 Here too, the reactivity of the imidazol-2-
ylphosphane ligands resembles the reactivity of imidazolylpho-
sphanes after protonation of the imine N atom (Scheme 7).
’ CONCLUSION
We have screened series of imidazole-based PN ligands in
the gold(I) catalyzed three component coupling of aldehydes,
amines and terminal alkynes as well as in the Markovnikov hydra-
tion of terminal alkynes. In homogeneous solution the conver-
sions to the respective propargylamine was considerably higher
than under biphasic conditions. Complex [(2c)AuCl] shows the
highest conversion of 95% with a catalyst loading of only 0.5 mol %.
Under biphasic conditions a 10-fold higher catalyst loading has to
be used for high activity, which is a result of decomposition of the
gold(I) complexes to the corresponding bis(NHC) gold(I)
complexes. Without any solvent the conversion rates using
(2a)AuCl and (2b)AuCl are slightly higher (7%). The conver-
sion using (2c)AuCl is with 30% considerably higher. On the
basis of the results we distinguished that connectivity of the
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Inorganic Chemistry
ARTICLE
density caused by disordered solvent molecules was treated with the
program utility SQUEEZE of the Platon program suite.34 Suitable
restraints were applied. The cif files can be found in the Supporting
Information. Additionally, CCDC 816459 and 816460 contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
72 h. The resulting precipitate was collected by filtration and dried in
vacuo. Yield: 1.1 g (52%). 1H NMR (MeOD-d4): δ = 7.27 À 7.35 (m,
9H). 31P{1H} NMR (MeOD-d4): δ = À46 (s). ESI+ (CH3OH): m/z
(%) = 243 [L]+. C12H11N4P CH3OH (274.26): calcd C 56.93, H 5.51,
3
N 20.43; found C 56.4, H 5.6, N 20.7.
2-Phenylimidazol-4(5)-yldiphenyl Phosphane (4-MIPPh, 4).
1-Methoxymethyl-2-phenylimidazole (1.5 g, 8.0 mmol) was placed in
a Schlenk tube equipped with a magnetic stirring bar and dissolved in
dry thf (100 mL). At À78 °C tert-butyllithium (5.3 mL, 1.6 M in
hexane) was added slowly to the solution, which turned deep red. The
reaction mixture was stirred at À78 °C for 1 h until the diphenylchlor-
ophosphane (1.6 mL, 8.0 mmol) was slowly added to the solution,
which turned yellow and is then was stirred overnight at room
temperature. The solvent was removed and the residue is dissolved in
with ammonia-saturated dichloromethane. This solution was stirred
overnight and the white solid was filtered off. The solvent was removed
from the filtrate and the residue dissolved in acetone/water (10: 1) and
2 mL of concentrated hydrochloric acid was added. The mixture was
refluxed for 4 h, all volatiles were removed in vacuo and the residue
dissolved in a minimum amount of ethanol and sodium hydroxide
solution added. The precipitate was filtered off, washed with diethyl
[(5)2Au3Cl3]. A dichloromethane solution (10 mL) of 4-MIPtBu (5)
(77 mg, 0.25 mmol) and [(tht)AuCl] (120 mg, 0.374 mmol) were
stirred for 4 h at ambient temperature. The solution was concentrated in
vacuo to 1/10th and the product precipitated by addition of diethyl
ether. Yield: 0.11 g (68%). 1H NMR (200 MHz, 296 K, CDCl3):
δ = 1.40 (s, 18H, CH3), 6.83 (s, 2H, Him), 7.62 À 7.73 (m, 20H, HPh),
13.52 (br, 2H, NH). 31P{1H}-NMR (81 MHz, 296 K, CDCl3): δ = 18
(s). MALDI TOF (DIT, CHCl3): m/z = 541 [LAuCl]+, 1277
[L2Au3Cl2]+. C38H42N4P2Au3Cl3 2 C4H10O (1462.23): calc. C
3
37.79, H 4.27, N 3.83; found C 37.8, H 4.2, N 3.8.
[(3c)2Au4Cl4]. A dichloromethane solution (10 mL) of 4-TIPiPr
(3c) (70 mg, 0.19 mmol) and [(tht)AuCl] (128 mg, 0.399 mmol) were
stirred for 17 h at ambient temperature. All volatiles were removed in
vacuo, the oily residue dissolved in dichloromethane (2 mL) and
the product precipitated by addition of diethyl ether. Yield: 0.11 g
1
ether, and dried in vacuo. Yield: 1.39 g (53%). H NMR (200 MHz,
1
CDCl3): δ = 3.23 (s, 3H, OCH3), 5.40 (d, J = 1.5 Hz, CH2), 6.86 Hz (s,
Him), 7.33À7.85 (m, 15H, Ph). 31P{1H} NMR (81 MHz, CDCl3): δ =
À34.0. ESI MS (methanol): m/z (rel. int.) = 386 (100) [M+NaCl]+,
(67%). H NMR (200 MHz, 296 K, MeOD-d4): δ = 1.36À1.50 (m,
3
36H, CH(CH3)2), 3.15 (sept, JHH = 6.85 Hz, 2H, CH(CH3)2(free)),
3.66 (sept, 3JHH = 7 Hz, 4H, CH(CH3)2(coord.)), 7.66 (s, 2H, Him(free)),
7.72 (s, 4H, Him(coord.)). 31P{1H} NMR (81 MHz, 296 K, MeOD-d4):
δ= À14 (s). ESI+ (CH3OH): m/z= 556 [LAu]+, 1110 [L2Au2ÀH]+, 1306
[L2Au3À2H]. MALDI TOF (DIT, CHCl3): m/z = 591 [LAuCl+H]+,
913 [L2Au]+, 11109 [L2Au2ÀH]+, 1305 [L2Au3À2H]+. C36H54-
329 (15) [M + H]+. C21H17N2P C2H5OH (374.42): calcd C 73.78, H,
3
6.19, N 7.48; found C 73.85, H 5.50, N 7.02.
2-tert-Butylimidazol-4(5)-yldiphenyl phosphane (4-MIPtBu, 5).
1-Methoxymethyl-2-tert-butylimidazole (1.3 g, 8.0 mmol) was placed in
a Schlenk tube equipped with a magnetic stirring bar and dissolved in
dry thf (100 mL). At À78 °C tert-butyllithium (5.3 mL, 1.6 M in
hexane) was added slowly to the solution, which turned deep red. The
reaction mixture was stirred at À78 °C for 1 h until the diphenylchlor-
ophosphane (1.6 mL, 8.0 mmol) was slowly added to the solution,
which turned yellow and was then stirred overnight at room tempera-
ture. The solvent was removed and the residue was dissolved in
ammonia-saturated dichloromethane. This solution was stirred over-
night, and the white solid was filtered off. The solvent was removed
from the filtrate and the residue dissolved in acetone/water (10: 1), and
2 mL of concentrated hydrochloric acid was added. The mixture was
refluxed for 4 h; all volatiles were removed in vacuo, and the residue was
dissolved in a minimum amount of ethanol and sodium hydroxide
solution was added. The precipitate was filtered off, washed with diethyl
N12P2Au4Cl2 1/2C4H10O (1611.19): calcd C 28.30, H 3.69, N
3
10.42; found C 28.3, H 4.2, N 10.9.
Gold-Catalyzed Three-Component Coupling Reactions. In
a Schlenk tube, under N2, the appropriate amount of the catalyst
was weight and phenyl acetylene (160 μL, 1.60 mmol), benzaldehyde
(100 μL, 1.00 mmol), and piperidine (110 μL, 1.10 mmol) were added.
1
Without purification the reaction mixture was analyzed by H NMR
spectroscopy.
Catalytic Hydration Reactions. Method a Halide Abstraction in
Situ: The gold complex (0.01 mmol, 5 mol % catalyst loading) was
dissolved in acetone (0.4 mL) and a solution of AgOTf (1 equiv) in
acetone (0.1 mL) is added. A white precipitate formed and the solution
turned yellow. Degassed 1-octyne (30 μL) and water (18 μL) were
added and the reaction mixture stirred at 60 °C in an oil bath. The
progress of the reaction was monitored via GC. Method b: The gold
chlorido compound was dissolved in dichloromethane and a solution of
AgOTf (1 equiv) in dichloromethane was added. The mixture was
stirred in a dark Schlenk tube at room temperature for 1 h and
precipitated AgCl was filtered off. Removal of most of the solvent under
vacuum and addition of diethyl ether resulted in the respective gold
complex as a fine white powder, which was dried in vacuo. The so-
obtained gold compound (0.01 mmol, 5 mol % catalyst loading) was
dissolved in acetone (0.4 mL) and degassed 1-octyne (30 μL) and water
(18 μL) were added to the solution. The reaction mixture was stirred at
60 °C in an oil bath. The progress of the reaction was monitored via GC.
Bisimidazol-2-ylphenyl Phosphane, (2-BIPH, 1b). A solution
of n-butyl lithium in n-hexane (1.6 M, 12 mL, 19 mmol) was added
dropwise to a solution of 3.0 g (18 mmol) of 1-diethoxymethylimidazole
in diethyl ether (150 mL) at À78 °C. The reaction mixture was stirred at
À40 °C for 1 h and then was cooled to À78 °C and PCl3 (1.56 g, 8.72
mmol) was added. The reaction mixture was stirred at À78 °C for 1 h
and at ambient temperature overnight. Concentrated ammonia solution
(5 mL) was added, the phases separated, the organic phase was collected
and all volatiles were removed in vacuo. The oily residue was dissolved in
100 mL acetone/water (10:1) and stirred at ambient temperature for
1
ether, and dried in vacuo. Yield: 1.06 g (43%). H NMR (200 MHz,
CDCl3): δ = 1.34 (s, 9H, CH3), 6.91 Hz (s, Him), 7.25À7.35 (m, 10H,
Ph). 31P{1H} NMR (81 MHz, CDCl3): δ = À31.0. ESI+ (methanol):
m/z (%) = 309 [M+H]+ (100), 249 [M-tBu]+ (95). C19H21N2P
5/3H2O (308.36): calcd C 67.44, H, 7.23, N 8.27; found C 67.49,
H 6.90, N 8.14.
3
General Procedure for the Synthesis of Gold(I) Complexes
(L)AuCl. To a stirred solution of (tht)AuCl was added the ligand
solution in a convenient solvent. The reaction solution was stirred for at
least 4 h at room temperature. The resulting solution was concentrated
in vacuo and diethyl ether was layered to precipitate white powders. The
product precipitate was filtered and dried in vacuo.
(2-MIPH)AuCl, (1a)AuCl. A solution of (tht)AuCl (80 mg,
0.25 mmol) in CH2Cl2 (5 mL) was added to a stirred solution of 2-MIPH
(63 mg, 0.25 mmol) in CH3OH (5 mL). Yield: 98 mg (81%). 1H NMR
(MeOD-d4): δ = 7.51À7.66 (m, 12H, HPh/im). 31P{1H} NMR (MeOD-
d4): δ = 16 (s). ESI+ (CH3OH): m/z (%) = 898 [(LAuCl)2]+. MALDI
MS (CH3OH): m/z (%) = 485 [LAuCl]+, 897 [(LAuCl)2]+. EI MS (Pt,
180 °C): m/z (%) = 251 [L]+, 484 [LAu]+, 895 [L2Au2À2H]+.
C15H13N2PAuCl 1/2CH2Cl2 (527.14): calcd C 35.32, H 2.68, N 5.31;
3
found C 35.4, H 2.4, N 5.1.
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Inorganic Chemistry
ARTICLE
(2-BIPH)AuCl, (1b)AuCl. A solution of (tht)AuCl (100 mg, 0.31
mmol) in CH2Cl2 (5 mL) was added to a stirred solution of 2-BIPH
(76 mg, 0.31 mmol) in CH3OH (10 mL). Yield: 0.11 g (74%). 1H NMR
(MeOD-d4): δ = 7.51À7.73 (m, 9H, Him/Ph). 31P{1H} NMR (MeOD-
d4):δ=À4(s). ESI+ (CH3OH): m/z(%) =475[LAuCl]+, 877[(LAu)2]+.
MALDI MS (CH3OH): m/z (%) = 440 [LAu]+, 878[(LAu)2]+.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: (+)49 211 8112287. E-mail: peter.kunz@uni-duesseldorf.de.
’ REFERENCES
C12H11N4PAuCl 1/4CH2Cl2 (495.87): calc. C 29.67, H 2.34, N
3
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11.30; found C 30.1, H 2.7, N 10.9.
(2-TIPH)AuCl, (1c)AuCl. A solution of (tht)AuCl (61 mg, 0.19 mmol)
in CH2Cl2 (5 mL) was added to a stirred suspension of 2-TIPH (44 mg,
0.19 mmol) in acetone (15 mL). The mixture was stirred for 72 h and
the resulting precipitate was filtered, washed with diethylether and dried
under vacuo. Yield: 64 mg (73%). 1H NMR (dmso-d6): δ = 7.41 (s, 6H,
Him), 13.24 (br, NH). 31P{1H} NMR (dmso-d6): δ = À20 (s). MALDI
MS (CH3OH): m/z (%) = 333 [(C3H4N2)Au]+, 465 [LAuCl]+.
C9H9N6PAuCl 1/2 CH2Cl2 (507.07): calcd C 22.50, H 1.99, N 16.57;
3
found C 22.1, H 1.8, N 16.3.
(4-BIPiPr)AuCl, (3b)AuCl. A solution of (tht)AuCl (60 mg, 0.19
mmol) in CH2Cl2 (5 mL) was added to a stirred solution of 4-BIPiPr (61
1
mg, 0.19 mmol) in CH3OH (5 mL). Yield: 93 mg (88%). H NMR
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(MeOD-d4): δ = 1.37 (d, J = 6.89 Hz, 12H, CH(CH3)2), 3.18À3.28
(sept, 2 H, H2), 7.52À7.80 (m, 7H, HPh/im). 31P{1H} NMR (MeOD-
d4): δ = À5 (br). ESI+ (CH3OH): m/z (%) = 524 [LAu]+, 1046
[(LAu)2]+. MALDI MS (CH3OH): m/z (%) = 559 [LAuCl]+, 1045
[(LAu)2]+, 1081 [(LAu)2Cl]+. C18H23N4PAuCl (558.80): calcd C
38.69, H 4.15, N 10.03; found C 38.9, H 3.9, N 10.3.
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(4-MIPNMe)AuCl, (6)AuCl. A solution of (tht)AuCl (96 mg, 0.3
mmol) in CH2Cl2 (5 mL) was added to a stirred solution of 4-MIPNMe
(80 mg, 0.3 mmol) in CH2Cl2 (5 mL). Yield: 99 mg (66%). 1H NMR
(CDCl3): δ = 3.73 (s, 3H, NCH3), 7.35À7,78 (m, 7H, HPh/im). 31P{1H}
NMR (CDCl3): δ = 11 (s). MALDI MS (CH3OH): m/z (%) = 463
[LAu]+, 499 [LAuCl], 729 [L2Au]+, 961 [L2Au2Cl]+, 1193
[L2Au3Cl2]+. C16H15N2PAuCl 1/2 CH2Cl2 1/4 C4H10O (559.70):
3
3
calcd C 37.55, H 3.33, N 5.01; found C 37.6, H 3.2, N 4.6.
(5-MIPNMe)AuCl, (7)AuCl. A solution of (tht)AuCl (100 mg, 0.31
mmol) in CH2Cl2 (5 mL) was added to a stirred solution of 5-MIPNMe
(83 mg, 0.31 mmol) in CH2Cl2 (5 mL). Yield: 90 mg (58%). 1H NMR
(CDCl3): δ = 3.81 (s, 3H, NCH3), 6.73 (br, 1H, Him), 7.55À7.73 (m,
10H, Hph), 7.79 (s, 1H, Him). 31P{1H} NMR (dmso-d6): δ = 11 (s).
MALDI MS (CH3OH): m/z (%) = 499 [LAuCl]+. C16H15N2PAuCl
3
1.5 CHCl3 1 H2O (695.77): calcd C 30.21, H 2.68, N 4.03; found C
3
29.8, H 2.8, N 4.4.
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(4-MIPPh)AuCl, (4)AuCl. A solution of (tht)AuCl (20 mg, 0.062
mmol) in CH2Cl2 (2 mL) was added to a stirred solution of 4-MIPPh
(20 mg, 0.062 mmol) in CH2Cl2 (2 mL). Yield: 32 mg (93%). 1H NMR
(dmso-d6): δ = 7.43À8.14 (m, 16H, Ph+CH); 31P{1H}-NMR (dmso-
d6): δ = 11 (s). MALDI MS (CH3OH): m/z = 1317 [Au3L2Cl2]+, 1085
[Au2L2Cl]+, 560 [AuLCl]+, 524 [AuL]+. C21H17N2P1AuCl (560.77):
calcd C 41.92, H 2.97, N 4.33; found C 41.90, H 3.27, N 3.95.
(4-MIPtBu)AuCl, (5)AuCl. A solution of (tht)AuCl (20 mg, 0.062
mmol) in CH2Cl2 (2 mL) was added to a stirred solution of 4-MIPPh
(19 mg, 0.062 mmol) in CH2Cl2 (2 mL). Yield: 28 mg (89%). 1H NMR
(dmso-d6): δ = 1.32 (s, 9H, CH3), 7.56À7.75 (m, 11H, Ph + CH), 12.58
(bs, 1H, NH). 31P{1H} NMR (dmso-d6): δ = 11 (s). MALDI MS
(CH3OH): m/z = 1277 [Au3L2Cl2]+, 1241 [Au3L2Cl]+, 540 [AuLCl]+.
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C19H21N2PAuCl 1.5CH2Cl2 (668.18): calc. C 36.85%; H 3.62%; N
3
4.19%; found C 36.7% H 3.7% N 3.7%.
’ ASSOCIATED CONTENT
S
Supporting Information. NMR spectra of the decomposi-
b
tion of (2c)AuCl and crystal data and structure refinement for
[(5)2Au3Cl2]Cl 3.5CHCl3 and [(3c)2Au4Cl2]Cl2. This material
3
is available free of charge via the Internet at http://pubs.acs.org.
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ARTICLE
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